1. Field
The following description relates to a rechargeable lithium battery.
2. Description of the Related Art
Rechargeable lithium batteries have recently drawn attention as a power source for small portable electronic devices. The batteries use an organic electrolyte and thereby have twice the discharge voltage of a conventional battery using an alkali aqueous solution, and accordingly have high energy density.
Various carbon-based materials such as artificial graphite, natural graphite and hard carbon have been used as a negative active material of a rechargeable lithium battery. All of these materials can intercalate and deintercalate lithium ions.
Lithium-transition element composite oxides that are capable of intercalating lithium, such as LiCoO2, LiMn2O4, LiNi1-xCoxO2 (0<x<1) and the like, have been researched for use as a positive active material of a rechargeable lithium battery.
Among the positive active materials, a spineltype manganese-based positive active material is easily synthesized at a relatively low cost, has better thermal stability compared to other active materials during overcharge and contains fewer environmental contaminants. However, a disadvantage of the spineltype Mn-based positive active material is its comparatively low capacity. In addition, Mn3+ is eluted from the surface of the Mn-based positive active material during the charge and discharge process, which causes a disproportionation reaction (2Mn3+→Mn4++Mn2+) on the surface of the Mn-based positive active material. This results in defects in the positive active material. The disproportionation reaction is more actively carried out during contact with an electrolyte, particularly at high temperatures. Due to the disproportionation reaction, eluted Mn ions break the positive active material, degrade the structural stability, and decrease the capacity. Specifically, the cycle life of the battery is deteriorated at high temperatures.
Several attempts have been made to control the manganese-eluting characteristics of the positive active material in order to improve the high temperature storage and the high temperature cycle life characteristics. One method is to partially substitute the manganese with other transition metals such as nickel, cobalt or the like. Alternatively, it has been suggested that since HF, produced by the hydrolysis of LiPF6, accelerates the elution of manganese, a material for neutralizing HF should be added to the electrolyte. A similar method is to coat the surface of the positive active material with lithium cobalt oxide. However, satisfactory improvements in the characteristics of a manganese-based active material have proven difficult to achieve using these methods.